The present invention relates to a method of treating molten aluminum by removing hydrogen gas and nonmetallic inclusions therefrom.
The term "aluminum" as used herein and in the claims includes pure aluminum and all aluminum alloys. Further the term "inert gas" is used herein as including argon gas, helium gas, krypton gas and xenon gas in the Periodic Table, and also including nitrogen gas which is inert to aluminum.
Molten aluminum before casting contains dissolved hydrogen gas, aluminum and magnesium oxides and like nonmetallic inclusions as undesirable impurities. Hydrogen gas and nonmetallic inclusions, when contained in molten aluminum, could create defects in the ingot obtained from the melt and the product prepared from the ingot. Accordingly, hydrogen gas and nonmetallic inclusions must be removed from molten aluminum.
Hydrogen gas and nonmetallic inclusions are removed from molten aluminum conventionally by introducing into the molten aluminum an inert gas, halogen gas such as chlorine gas, or halogen compound gas such as Freon in the form of bubbles. However, the water contained in the atmosphere (in an amount of up to about 30 mg/liter during summer or up to about 5 mg/liter during winter in Osaka, Japan) poses the problem that aluminum reacts with the water in the atmosphere at the surface of molten aluminum (2Al+3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +3H.sub.2), permitting the resulting hydrogen to penetrate into the molten aluminum. The surface of molten aluminum at rest is usually covered with a compact film of aluminum oxide which prevents the aluminum from reacting with the water in the atmosphere. Nevertheless, when an inert gas, chlorine gas or like treating gas is introduced in the form of bubbles into molten aluminum, the bubbles rising to the surface of the melt disturb the surface and break the aluminum oxide film covering the surface to expose the aluminum melt surface to the atmosphere. Consequently, the water in the atmosphere reacts with the aluminum to evolve hydrogen gas, which penetrates into the molten aluminum before a fresh oxide film is formed at the broken portion.
To solve the problem, it has been proposed to introduce the treating gas into molten aluminum as placed in a sealed container, with its interior space above the melt surface filled with an inert gas to a pressure higher than atmospheric pressure (U.S. Pat. No. 3,870,511). However, this method requires an expensive device for holding the container sealed, while even if the container is of sealed structure, some air inevitably ingresses into the container through an inlet for molten aluminum and through a minute clearance between the closure and the container main body. We have substantiated by experiments that even if the interior atmosphere above the melt surface has a water content of as low as about 0.5 mg/liter due to the ingress of some air, the water reacts with the molten aluminum to evolve hydrogen, with the result that it is impossible to achieve a satisfactory hydrogen gas removal efficiency, i.e., to reduce the amount of hydrogen gas in the melt to about 0.10 cc per 100 g of the melt.
Moreover, it is difficult for the conventional methods to effectively remove hydrogen gas from molten aluminum having a high purity of at least 99.9 wt. %.